1. Field of the Invention
This invention pertains to microfiber products and methods of making them. More specifically it relates to such products in tow or tube form containing a mixture of microfibers and larger diameter, short fibers and useful, for example, as tube or plug filters, beauty coils, tampons and for many other applications. Such structures will have a generally rounded cross section although it may not be perfectly circular. The entanglement of the microfibers provides an integrated network while the selection of the larger diameter, short fibers is made to impart desired porosity, loft, absorbency, strength or other properties as well as for increased economy. Methods for forming microfibers by meltblowing are preferred as is the use of a separate airstream to mix the larger diameter, short fibers with the microfibers. Such structures will find broad utility wherever a rope, tow, or tube of soft, porous absorbent fibers is desired.
2. Description of the Prior Art
Formation of microfibers by meltblowing is well-known and described, for example, in U.S. Pat. No. 3,595,245 to Buntin, Harding, Keller and Murdock dated 27 July 1971. That patent describes meltblowing of a tow using a cylindrical meltblowing die to form essentially continuous fibers that are looped back and forth on a forming rod and built up to the desired roving diameter. It is also suggested in the patent that other ingredients may be incorporated into the roving by injection into one or more of the airstreams although no specific details of such a process or product are described. It is further known as described in U.S. Pat. No. 4,100,324 to Anderson, Sokolowski and Ostermeier dated 11 July 1978 and U.S. Pat. No. 4,118,531 to Hauser dated 03 Oct. 1978 to form webs including microfibers having mixed therein through a separate airstream staple or other fibers. There is, though, no suggestion in these patents that a circular roving may be formed of such mixtures nor of a method for forming such a roving. U.S. Pat. No. 4,357,379 to Sloan, Wright, and Morie dated 02 Nov. 1982 describes a modification of the meltblowing process to form a rod by a stuffing technique including meltblowing into a horn and withdrawing the fibers in a manner such that they are oriented generally in a radial direction of the formed rod. Examples of uses for rovings or tows of meltblown microfibers are disclosed in the identified patents, and, in addition, U.S. Pat. No. 3,219,043 to Ehrlich and Teague dated 23 Nov. 1965 and U.S. Pat. No. 4,390,031 to Berger dated 28 June 1983 disclose filter applications for thermoplastic fiber cylindrical tows.
In spite of these teachings, it remains desired to form tows or tubes of microfibers with improved properties for tube or plug filters, e.g. cigarette filters, beauty coils, tampons and other applications. In particular, such microfiber tows having improved porosity, increased strength, absorbency, and other characteristics as well as potentially reduced costs are considered desirable. It is, moreover, desired to have improved methods for forming such tow or tube materials.